Initialisation Scripts

A collection of initialisations are performed through running the
script /etc/init.d/rcS as specified in the initialisation
table (/etc/inittab) with the line:

si::sysinit:/etc/init.d/rcS

The particular initialisations are performed through scripts located
in the /etc/rcS.d/ folder. The names of these scripts are all
of the form SNNmmmm where NN is numeric and mmmm
is the name of some Debian package or application, such as
setserial and hwclock. For example, you may find
/etc/rcS.d/S30setserial and
/etc/rcS.d/S50hwclock.sh.

The script /etc/init.d/rcS will ensure all of these scripts
are run, one after another, in numeric and alphabetic order. Order is
often important. Any script ending with .sh is sourced as a
shell script and the others are run as commands. Irrespective of how
the script is invoked it is passed the parameter start
indicating that the service that the initialisation script represents
is to be started in some sense. This is the meaning of the single
S in the scrip file names and in the name of the rcS
script.

You can add your own scripts into /etc/init.d/ folder and
then link them appropriately to the particular run level directories.
The update-rc.d command should be used to do this for you.
Suppose you want to set up a firewall each time you boot the machine.
You may create an executable script file called
/etc/init.d/myfirewall (see Section 63.11 for
sample contents). Then add this to the appropriate folders with: